Ideally MUX/DEMUX systems perform consistently in spite of small fluctuations in laser wavelength, which requires that the MUX/DEMUX be designed with flat passbands in the frequency domain.
Numerous designs exist for both arrayed-waveguide grating (AWG) and echelle-grating etched waveguide spectrometers, which are used for optical MUX/DEMUX or optical channel monitors/performance monitors (OCM/OPM) in the field of optical telecommunications. Conventionally, flat-passband performance of the spectrometer unit is achieved at the expense of higher insertion loss, by degrading the shape of the passband from the ideal narrow-peaked Gaussian bandshapes, which are common to spectrometers in waveguide based devices. The bandshape is degraded by widening the optical aperture at the entrance to or exit of the spectrometer unit, and/or by introducing aberrations, e.g. de-focus, coma, spherical, to the interference element. Even for an ideal design, a flat passband top with sharp band cutoffs will come only at the expense of spectrometer transmission. Furthermore, the passband flattening will not result in temporal narrowing in the existing designs.
With conventional grating-based devices, such as the ones disclosed in U.S. Pat. No. 6,298,186 issued Oct. 2, 2001 to Jian-Jun He and U.S. Pat. No. 6,188,818 issued Feb. 13, 2001 to Han et al, a flat passband performance can only be achieved by sacrificing transmission at the peak of each channel. Moreover, there is no shortening of impulse in the time domain accompanying the flattening of the passband in the frequency domain.
Accordingly, an object of the present invention is to overcome the shortcomings of the prior art by providing a MUX/DEMUX including a pair of gratings to be used in sequence such that the emission from the first grating achieves a cyclic offset of incidence angle into the second grating of the system.